Abstract: In studies of multi-cluster hydraulic fracture propagation patterns, traditional research has emphasized inter-cluster competitive and unbalanced propagation caused by stress shadowing effects. However, theoretical analysis and field monitoring reveal that unbalanced propagation should also include the concept of spatial asymmetry, especially as fracturing processes evolve toward dense-cluster spacing and extreme limited-entry perforation techniques. The spatially asymmetric propagation of hydraulic fractures significantly affects the identification of effective stimulated volume, assessment of inter-well interference, and optimization of extreme limited-entry perforation techniques. Taking the Lianggaoshan Formation shale oil in the Fuxing block as an example, this study established an asymmetric propagation model for multi-cluster fractures in horizontal wells with dense-cluster spacing using a three-dimensional discrete lattice simulation method. The model precisely characterized the spatial evolution of hydraulic fractures , analyzed sensitivity factors influencing symmetry, and proposed optimization strategies. The results showed that: (1) stress shadowing between fractures led to asymmetric propagation and complementary morphologies. As cluster density increased, asymmetric propagation became more pronounced, primarily in middle clusters, consistent with microseismic monitoring results. (2) As the pad fluid changed from low viscosity to gelled fluid, the asymmetry index showed a decreasing trend, and the equilibrium index showed a significant increasing trend. Increasing pad fluid viscosity enhanced fracture initiation and propagation uniformity while reducing asymmetry to some extent. However, severe fracture deflection occurred and complexity increased, limiting fracture length and requiring further scale enhancement. (3) Limited-entry perforation significantly improved initiation uniformity but exacerbated asymmetry, with severe deflection in middle-cluster fractures, particularly when cluster spacing was ≤6 m. However, when cluster spacing was ≥8 m, limited-entry perforation not only enhanced initiation uniformity but also effectively reduced the asymmetry of hydraulic fractures. In subsequent operations, based on full consideration of symmetry and uniformity, cluster spacing, fluid viscosity, and injection rate should be reasonably configured, supplemented by temporary plugging and limited-entry perforation techniques, to improve the stimulated volume of the hydraulic fracture network. This study provides a novel perspective on asymmetric fracture propagation for optimizing multi-cluster fracturing and perforation design in unconventional reservoirs.

Key words: shale, hydraulic fracturing, asymmetric propagation, three-dimensional discrete lattice method, stress shadow, Lianggaoshan Formation